Extreme ultraviolet lithography system, device, and method for printing low pattern density features

ABSTRACT

A lithography system includes a radiation source configured to generate an extreme ultraviolet (EUV) light. The lithography system includes a mask that defines one or more features of an integrated circuit (IC). The lithography system includes an illuminator configured to direct the EUV light onto the mask. The mask diffracts the EUV light into a 0-th order ray and a plurality of higher order rays. The lithography system includes a wafer stage configured to secure a wafer that is to be patterned according to the one or more features defined by the mask. The lithography system includes a pupil phase modulator positioned in a pupil plane that is located between the mask and the wafer stage. The pupil phase modulator is configured to change a phase of the 0-th order ray.

PRIORITY DATA

This application is a continuation application of U.S. patentapplication Ser. No. 16/220,324, filed Dec. 14, 2018, entitled “IMPROVEDEXTREME ULTRAVIOLET LITHOGRAPHY SYSTEM, DEVICE, AND METHOD FOR PRINTINGLOW PATTERN DENSITY FEATURES, which is a continuation application ofU.S. patent application Ser. No. 15/380,717, filed Dec. 15, 2016, nowU.S. Pat. No. 10,162,257, issued Dec. 25, 2018, entitled “IMPROVEDEXTREME ULTRAVIOLET LITHOGRAPHY SYSTEM, DEVICE, AND METHOD FOR PRINTINGLOW PATTERN DENSITY FEATURES,” the entire disclosures of each of whichare incorporated herein by reference in their entirety.

BACKGROUND

The semiconductor integrated circuit (IC) industry has experiencedexponential growth. Technological advances in IC materials and designhave produced generations of ICs where each generation has smaller andmore complex circuits than the previous generation. In the course of ICevolution, functional density (i.e., the number of interconnecteddevices per chip area) has generally increased while geometry size(i.e., the smallest component (or line) that can be created using afabrication process) has decreased. This scaling down process generallyprovides benefits by increasing production efficiency and loweringassociated costs. Such scaling down has also increased the complexity ofIC processing and manufacturing. For these advances to be realized,similar developments in IC processing and manufacturing are needed. Forexample, the need to perform higher resolution lithography processesgrows. One lithography technique is extreme ultraviolet lithography(EUVL). Other techniques include X-Ray lithography, ion beam projectionlithography, electron beam projection lithography, and multiple electronbeam maskless lithography.

The EUVL employs scanners using light in the extreme ultraviolet (EUV)region. EUV scanners provide the desired pattern on an absorption layer(“EUV” mask absorber) formed on a reflective mask. Currently, binaryintensity masks (BIM) are employed in EUVL for fabricating integratedcircuits. For EUV light, all materials are highly absorbing. Thus,reflective optics rather than refractive optics is used. A reflectivemask is used. However, the reflectance of EUV mask is very low. The EUVenergy is substantially lost on the optical path. The EUV energyreaching the wafer is much less. There are other issues including lowthroughput issue, especially for a via layer due to the lowtransmittance through the via.

Therefore, what is needed is the method for a lithography process andthe mask structure utilized in the method to address the above issues.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isnoted that, in accordance with the standard practice in the industry,various features are not drawn to scale. In fact, the dimensions of thevarious features may be arbitrarily increased or reduced for clarity ofdiscussion.

FIG. 1 is a flowchart of a lithography process constructed according toaspects of the present disclosure in various embodiments.

FIG. 2 is a block diagram of a lithography system for implementing amask structure constructed according to aspects of the presentdisclosure in one or more embodiment.

FIG. 3 is a diagrammatic perspective view of the lithography systemconstructed according to one embodiment.

FIG. 4 is a top view of a binary phase mask constructed according toaspects of the present disclosure in one or more embodiment.

FIGS. 5A and 5B are diagrammatic cross-sectional views of the binaryphase mask constructed according to aspects of the present disclosure intwo embodiments.

FIGS. 6-8 are a diagrammatic cross-sectional view of the secondreflective layer of FIG. 5A (Or 5B) constructed according to aspects ofthe present disclosure in various embodiments.

FIGS. 9A through 9C are diagrammatic top views of an illuminator used inthe lithography system of FIG. 3, constructed according to aspects ofthe present disclosure in various embodiments.

FIGS. 10A through 10C are diagrammatic top views of a pupil filter usedin the lithography system of FIG. 3, constructed according to aspects ofthe present disclosure in various embodiments.

FIGS. 11A and 11B are diagrammatic top views of a pupil filter used inthe lithography system of FIG. 3, constructed according to otherembodiments.

FIG. 12 illustrates the exposure light field distribution before thepupil filter constructed according to aspects of the present disclosurein one embodiment.

FIG. 13 illustrates the exposure light field distribution after thepupil filter constructed according to aspects of the present disclosurein one embodiment.

FIG. 14 is a schematic view of an integrated circuit (IC) patternconstructed according to aspects of the present disclosure in oneembodiment.

FIG. 15 is a schematic view of an image of the IC pattern of FIG. 14 onthe target using the BPM, constructed according to aspects of thepresent disclosure in one embodiment.

FIG. 16 is a schematic view of an image of the IC pattern of FIG. 14 onthe target using the BIM, constructed according to one embodiment.

FIG. 17 illustrates diagrammatically the mask error enhancement factor(MEEF) over the dimension on mask (DOM), constructed according tovarious embodiments.

FIG. 18 is a diagrammatic cross-sectional view of the binary phase maskwith an exemplary particle constructed according to aspects of thepresent disclosure in one embodiment.

FIG. 19 is a block diagram of a lithography process for implementing oneor more embodiments of the present invention.

FIG. 20 is a diagrammatic perspective view of a projection optics box(POB) employed in the lithography process for implementing one or moreembodiments of the present invention. Since a POB by reflective opticsis difficult to sketch, the equivalent refractive optics is used toillustrate the underlying principle.

FIG. 21 is a diagrammatic cross-sectional view of various aspects of oneembodiment of a mask blank at various stages of a lithography processconstructed according to aspects of the present disclosure.

FIG. 22 is a diagrammatic cross-sectional view of various aspects of oneembodiment of a mask at various stages of a lithography processconstructed according to aspects of the present disclosure.

FIGS. 23A through 23C are diagrammatic top views of an illuminator usedin the lithography system of FIG. 3, constructed according to aspects ofthe present disclosure in various embodiments.

FIGS. 24A through 24C are diagrammatic planar views of a pupil phasemodulator used in the lithography system of FIG. 3, constructedaccording to aspects of the present disclosure in various embodiments.

FIGS. 25A-25B illustrate the exposure light field distribution beforeand after the pupil phase modulator constructed according to aspects ofthe present disclosure in one embodiment.

FIGS. 26A through 26B are diagrammatic planar views of a pupil phasemodulator used in the lithography system of FIG. 3, constructedaccording to aspects of the present disclosure in various embodiments.

FIGS. 27, 28A, 28B, and 28C are diagrammatic cross-sectional views of aportion of a pupil phase modulator according to embodiments of thepresent disclosure.

FIGS. 29A-29B are diagrammatic planar and cross-sectional views of apupil phase modulator according to embodiments of the presentdisclosure.

FIGS. 30A-30B are diagrammatic planar and cross-sectional views ofanother pupil phase modulator according to embodiments of the presentdisclosure.

FIG. 31 is a flowchart of a lithography process constructed according toaspects of the present disclosure in various embodiments.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features. Specific examples ofcomponents and arrangements are described below to simplify the presentdisclosure. These are, of course, merely examples and are not intendedto be limiting. For example, the formation of a first feature over or ona second feature in the description that follows may include embodimentsin which the first and second features are formed in direct contact, andmay also include embodiments in which additional features may be formedbetween the first and second features, such that the first and secondfeatures may not be in direct contact. In addition, the presentdisclosure may repeat reference numerals and/or letters in the variousexamples. This repetition is for the purpose of simplicity and clarityand does not in itself dictate a relationship between the variousembodiments and/or configurations discussed.

Further, spatially relative terms, such as “beneath,” “below,” “lower,”“above,” “upper” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. The spatiallyrelative terms are intended to encompass different orientations of thedevice in use or operation in addition to the orientation depicted inthe figures. The apparatus may be otherwise oriented (rotated 90 degreesor at other orientations) and the spatially relative descriptors usedherein may likewise be interpreted accordingly.

FIG. 1 is a flowchart of a method 10 to perform a lithography process inan integrated circuit fabrication constructed according to aspects ofthe present disclosure in various embodiments. The method 10, thelithography system and the photomask used by the method 10 are describedwith reference to FIG. 1 and other figures.

Referring to FIG. 1, the method 10 includes an operation 12 by loadingto a lithography system 30 with a photomask (mask or reticle) 36. In thepresent disclosure, the mask 36 is designed to have phase shift and withtwo mask states. Therefore, the mask 36 is a phase shift mask with twophase states, therefore being referred to as binary phase mask (BPM).The lithography system 30 and the mask 36 are described belowrespectively.

FIG. 2 illustrates a block diagram of the lithography system 30 forperforming a lithography exposure process. The lithography system 30 isalso illustrated, in portion, in FIG. 3 in a schematic view. In thepresent embodiment, the lithography system 30 is an extreme ultraviolet(EUV) lithography system designed to expose a resist (or photoresist)layer by EUV. The resist layer is sensitive to the EUV radiation. TheEUV lithography system 30 employs a radiation source 32 to generate EUVlight, such as EUV light having a wavelength ranging from about 1 nm toabout 100 nm. In one particular example, the EUV radiation source 32generates a EUV light with a wavelength centered at about 13.5 nm.

The EUV lithography system 30 also employs an illuminator 34. In variousembodiments, the illuminator 34 includes various refractive opticcomponents, such as a single lens or a lens system having multiplelenses (zone plates) or reflective optics, such as a single mirror or amirror system having multiple mirrors in order to direct light from theradiation source 32 onto a mask 36. In the present embodiment where theradiation source 32 that generates light in the EUV wavelength range,reflective optics is employed. Refractive optics, however, can also berealized by zoneplates for example. In the present embodiment, theilluminator 34 is operable to configure the mirrors to provide anoff-axis illumination (OAI) to illuminate the mask 36. In one example,the mirrors of the illuminator 34 are switchable to reflect EUV light todifferent illumination positions. In another embodiment, a stage priorto the illuminator 34 may additionally include other switchable mirrorsthat are controllable to direct the EUV light to different illuminationpositions with the mirrors of the illuminator 34. Accordingly, thelithography system 30 is able to achieve different illumination modeswithout sacrificing the illumination energy.

The EUV lithography system 30 also includes a mask stage 35 configuredto secure a photomask 36 (in the present disclosure, the terms of mask,photomask, and reticle are used to refer to the same item). The mask 36may be a transmissive mask or a reflective mask. In the presentembodiment, the mask 36 is a reflective mask such as described infurther detail below.

The EUV lithography system 30 also employs the POB 38 (projection opticsbox) for imaging the pattern of the mask 36 on to a target 40 (such as asemiconductor wafer) secured on a substrate stage 42 of the lithographysystem 30. The POB 38 may have refractive optics or reflective optics.The radiation reflected from the mask 36 (e.g., a patterned radiation)is collected by the POB 38. In one embodiment, the POB 38 may include amagnification of less than one (thereby reducing the patterned imageincluded in the radiation).

The structure of the mask 36 and the method making the same will befurther described later according to various embodiments. The maskfabrication process includes two operations: a blank mask fabricationprocess and a mask patterning process. During the blank mask fabricationprocess, a blank mask is formed by deposing suitable layers (e.g.,multiple reflective layers) on a suitable substrate. The blank mask ispatterned during the mask patterning process to have a design of a layerof an integrated circuit (IC). The patterned mask is then used totransfer circuit patterns (e.g., the design of a layer of an IC) onto asemiconductor wafer. The patterns can be transferred over and over ontomultiple wafers through various lithography processes. Several masks(for example, a set of 15 to 30 masks) may be used to construct acomplete IC. In general, various masks are fabricated for being used invarious processes.

The mask 36 incorporates phase-shifting mask (PSM) technique and isdesigned to achieve the enhanced illumination intensity when used withthe lithography system 30 and the method 10. In the present embodiment,the mask 36 is a binary phase mask. FIG. 4 illustrates a top view of themask 36 and FIG. 5A is a sectional view of the mask 36, constructedaccording to aspects of the present disclosure.

Referring to FIG. 4, the mask 36 includes a plurality of main features(main polygons) 80. The rest region without main patterns is referred toas field 82. A main polygon is a IC feature or a portion of the ICfeature that will be imaged to the target 40 (a wafer in the presentexample). In one example, the main feature 80 is an opening that definesa via in a via layer (or a contact in a contact layer) to be formed onthe semiconductor wafer. The pattern in the mask 36 defines the vialayer with a plurality of vias (or the contact layer with plurality ofcontacts). In another example, the main feature 80 is an opening thatdefines a cut feature for double or multiple patterning. The pattern inthe mask 36 defines a cut pattern with a plurality of cut featuresdesigned to form a circuit pattern (such as gates or metal lines) withone or more main patterns defined on corresponding mask by two or moreexposures. Double patterning as one example of the multiple patterningis further described to illustrate the cut pattern. During doublepatterning, a first mask defines main features (such as metal lines),and a second mask defines cut features where each cut feature break(cut) the corresponding main feature (such as one metal line) into twomain features (such as two metal lines) through a double patterningprocess. In yet another embodiment, the pattern in the mask 36 mayfurther include other features, such as optical proximity correction(OPC) features to enhance imaging effect and/or dummy features toimprove performance of other fabrication operations (such as CMP, andthermal annealing). In the present embodiment, the pattern density ofthe pattern on the mask 36 has a low pattern density, such as lower thanabout 25% in one example. In other example such as a pattern with areversed tone where the pattern density is calculated by thecomplimentary areas on the mask, the pattern density is greater than75%.

Referring to FIG. 5A, the mask 36 includes a mask substrate 84, such asa substrate made of low thermal expansion material (LTEM). In variousexample, the LTEM material includes TiO₂ doped SiO₂, or other lowthermal expansion materials with low thermal expansion. The masksubstrate 84 serves to minimize image distortion due to mask heating orother factors. In furtherance of the present embodiment, the masksubstrate 84 of the LTEM includes a suitable material with a low defectlevel and a smooth surface. In another embodiment, a conductive layermay be additionally disposed on back surface of the mask substrate 84for the electrostatic chucking purpose. In one example, the conductivelayer includes chromium nitride (CrN), though other compositions arepossible.

The mask 36 includes a reflective multilayer (ML) 86 disposed over themask substrate 84 on the front surface. The ML 86 is also referred to asa first reflective layer, to avoid confusion when another reflectivelayer to be introduced later. According to Fresnel equations, lightreflection will occur when light propagates across the interface betweentwo materials of different refractive indices. The reflected light islarger when the difference of refractive indices is larger. To increasethe reflected light, one may also increase the number of interfaces bydeposing a multilayer of alternating materials and let lights reflectedfrom different interfaces interfere constructively by choosingappropriate thickness for each layer inside the multilayer. However, theabsorption of the employed materials for the multilayer limits thehighest reflectivity that can be achieved. The ML 86 includes aplurality of film pairs, such as molybdenum-silicon (Mo/Si) film pairs(e.g., a layer of molybdenum above or below a layer of silicon in eachfilm pair). Alternatively, the ML 86 may include molybdenum-beryllium(Mo/Be) film pairs, or any suitable material that is highly reflectiveat EUV wavelengths. The thickness of each layer of the ML 86 depends onthe EUV wavelength and the incident angle. The thickness of the ML 86 isadjusted to achieve a maximum constructive interference of the EUV lightreflected at each interface and a minimum absorption of the EUV light bythe ML 86. The ML 86 may be selected such that it provides a highreflectivity to a selected radiation type and/or wavelength. In atypical example, the number of the film pairs in the ML 86 ranges from20 to 80, however any number of film pairs is possible. In one example,the ML 86 includes forty pairs of layers of Mo/Si. In furtherance of theexample, each Mo/Si film pair has a thickness of about 7 nm (a Mo filmof about 3 nm thick and a Si film of about 3 nm thick), with a totalthickness of 280 nm. In this case, a reflectivity of about 70% isachieved.

A capping layer may be formed above the ML 86 for one or more functions.In one example, the capping layer functions as an etch stop layer in apatterning process or other operations, such as repairing or cleaning.In another example, the capping layer functions to prevent oxidation ofthe ML 86. The capping layer may include one or more films to achievethe intended functions. In one example, the capping layer has differentetching characteristics from a second reflective layer 88, which will bedescribed later. In another example, the capping layer includesruthenium (Ru). In furtherance of the example, the capping layerincludes a Ru film with a thickness ranging from about 2 nm to about 5nm. In other examples, the capping layer may include Ru compounds suchas RuB, RuSi, chromium (Cr), Cr oxide, or Cr nitride. A low temperaturedeposition process may be chosen to form the capping layer to preventinter-diffusion of the ML 86.

The mask 36 further includes the second reflective layer 88 formed abovethe first reflective layer (the ML layer) 86. The second reflectivelayer 88 is designed (such as by composition, configuration andthickness) to reflect the EUV light without absorption or lestabsorption to avoid energy loss. The second reflective layer 88 isdesigned to further provide a phase shift to the reflected EUV lightrelative to the reflective EUV light from the first reflective layer 86.In the present embodiment, the phase difference of the reflected EUVlights from the first and second reflective layers is substantially 180°or close to 180° such that to achieve the enhanced exposure intensityduring the lithography exposure process. Accordingly, the secondreflective layer 88 functions as a phase shift and is a phase shiftmaterial layer.

The second reflective layer 88 is patterned according to an integratedcircuit pattern having various main features 80. In one embodiment wherethe capping layer is present, the second reflective layer 88 is formedabove the capping layer.

Thus, the mask 36 is a phase shift mask with two states, a first maskstate and a second mask state. Both mask states substantially reflectthe EUV light but with a phase difference (180° in the presentembodiment). The first mask state is defined in the regions of the firstreflective layer 86 within the openings of the second reflective layer88, such as the main feature 80 being defined in the first mask state.The second mask state is defined in the regions of the second reflectivelayer 88, such as the field 82 being defined in the second mask state.Thus, the mask 36 is a binary phase mask or BPM. The IC pattern with lowpattern density is defined below according to various embodiments. Inthe BPM 36, the first area S1 associated with the first mask state andthe second area S2 associated with second mask state have a ratio in acertain range. In one embodiment, the ratio S1/S2 is less than about1/3, such as the mask of the first type illustrated in FIG. 5A as oneexample. In an alternative embodiment, the ratio S1/S2 is greater thanabout 3, such as the mask of the second type illustrated in FIG. 5B asan example.

In a conventional binary intensity mask (BIM), the patterned layer is anabsorption layer. Different from the BIM, the absorption layer isreplaced by a phase shift material layer in the BPM.

FIG. 5A illustrates the mask 36 of the first type. The mask 36 of thesecond type is illustrated in FIG. 5B. The mask 36 inn FIG. 5B issimilar to the mask 36 in FIG. 5A. Both include the first and secondreflective layers 86 and 88. However, in FIG. 5B, the main feature 80 isdefined in the second mask state and the field 82 is defined in thefirst mask state. Particularly, in FIG. 5B, the field 82 is defined inthe region of the first reflective layer 86 within the opening of thesecond reflective layer 88 and the main feature 80 is defined within thesecond reflective layer 88. Since the first and second mask states aredifferent from each other only in phase in ideal situation. This reversetone mask may result the same image during the subsequent exposureprocess. In other situations where the first and second mask states mayhave different absorptions, the exposure process still can be tuned withillumination mode and the pupil filter to enhance the exposure intensitywith reduced the energy loss.

The second reflective layer 88 may have various compositions andconfigurations according to various embodiments. The second reflectivelayer 88 may be different from the first reflective layer 86 in terms ofcomposition and configuration in addition to that the second reflectivelayer 88 is patterned according to an IC layout.

In the present embodiment, the thickness of the second reflective layer88 is less than that of the first reflective layer 86. Thus, the stepheight of the second reflective layer 88 after being patterned isreduced in order to eliminate or reduce the shadow effect. In apreferred embodiment, the second reflective layer 88 has a thicknessless than 70 nm, in order to effectively reduce the shadow effect for ICwith small feature sizes, such as feature size of 20 nm.

One embodiment of the second reflective layer 88 is illustrated in FIG.6 in a sectional view. The second reflective layer 88 includes a singlemolybdenum (Mo) film 92 with a thickness ranging from about 40 nm toabout 48 nm. The total thickness of the second reflective layer 88 issame to the thickness of the single Mo film 92 since the secondreflective layer 88 includes only this Mo film. Thus designed secondreflective layer 88 has a thickness less than about 50 nm, andfurthermore is able to provide phase shift of about 180° andreflectivity of about 0.776 to the EUV radiation. Stated differently,the designed second reflective layer 88 provides reflectivity of about−0.776 where the sign “−” stands for 180° phase shift. Accordingly, thereflected EUV intensity is about 60% and the energy loss is about 40%.

Another embodiment of the second reflective layer 88 is illustrated inFIG. 7 in a sectional view. The second reflective layer 88 includesmultiple films. Particularly, the second reflective layer 88 includesfive silicon films 102, 104, 106, 108 and 110; and five Mo films 112,114, 116, 118 and 120 configured such that two adjacent Mo filmssandwich a silicon film and two adjacent silicon film sandwich a Mofilm. The second reflective layer 88 may further include a capping layer122 disposed on the top silicon film 110. In the present embodiment, thesilicon film 102 has a first thickness T1, the silicon films 104, 106and 108 have a same second thickness T2 greater than the first thicknessT1 and the silicon Mo film 110 has a third thickness T3 less than thefirst thickness T1. In the present embodiment, the Mo layer 112 has afourth thickness T4 less than the third thickness T3, and the Mo films114, 116, 118 and 120 have a same fifth thickness T5 greater than thesecond thickness T2. Those films are tuned collectively to have athickness less than 70 nm, phase shift of about 180°, and a reflectivityto the EUV radiation such that the energy loss is less than about 40%.

In the present example, the thickness parameters T1, T2, T3, T4 and T5are about 4 nm, about 4.3 nm, about 2.6 nm, about 1 nm and about 10.1nm, respectively, each being within about 20% of the nominal value. Forexample, the first thickness T1 ranges from about 4× (1+20%) nm to about4× (1−20%) nm.

The capping layer 122 may be similar to the capping layer describedabove in FIG. 5A. In one embodiment, the capping layer 122 includes a Rufilm. In furtherance of the embodiment, the capping layer 122 includes aRu film with a thickness ranging between about 2 nm and about 5 nm. Inother examples, the capping layer 122 may include Ru compounds such asruthenium boron (RuB), ruthenium silicon (RuSi), chromium (Cr), Croxide, or Cr nitride.

In the present example, the reflectivity of the second reflective layer88 is about 0.867 or about −0.867 where the sign “−” stands for 180°phase shift. Accordingly, the reflected radiation energy intensity isabout 75% and the radiation energy loss is about 25%.

Another embodiment of the second reflective layer 88 is illustrated inFIG. 8. In this embodiment, the second reflective layer 88 issubstantially similar to the first reflective layer 86 in terms ofcomposition and configuration. The second reflective layer 88 alsoincludes a multiple reflective layer similar to that of the firstreflective layer 86. For example, the second reflective layer 88includes a plurality of film pairs (“m1” and “m2”), such as Mo/Si filmpairs (e.g., a layer of molybdenum above or below a layer of silicon ineach film pair). In the present embodiment, the second reflective layer88 includes about 15 pairs of Mo/Si film to achieving 180° phase shift.In other examples, the second reflective layer 88 alternatively includemolybdenum-beryllium (Mo/Be) film pairs, or any suitable material thatis highly reflective at EUV wavelengths. The thickness of each layer ofthe second reflective layer 88 depends on the EUV wavelength and theincident angle.

Each of the layers (such as films 102-122 in FIG. 7) may be formed byvarious methods, including physical vapor deposition (PVD) process, aplating process, a chemical vapor deposition (CVD) process, ion beamdeposition, spin-on coating, metal-organic decomposition (MOD), and/orother methods known in the art.

The second reflective layer 88 may be patterned by a suitable patterningtechnique. A patterning process may include resist coating (e.g.,spin-on coating), soft baking, mask aligning, exposure, post-exposurebaking, developing the resist, rinsing, drying (e.g., hard baking),other suitable processes, and/or combinations thereof. An etchingprocess is followed to remove a portion of the patterned reflectivelayer 88.

The mask 36 includes two mask states, 80 and 82. The mask 36 alsoincludes a conductive layer 126 disposed on back surface of the masksubstrate 84 for the electrostatic chucking purpose. In one example, theconductive layer 126 includes chromium nitride (CrN), though othercompositions are possible. The mask 36 further includes a capping layer128 formed between the first and second reflective layers 86 and 88. Thecapping layer 128 may include one or more films. In one example, thecapping layer 128 has different etching characteristics from a secondreflective layer 88. In another example, the capping layer 128 includesRu. In furtherance of the example, the capping layer 128 includes a Rufilm with a thickness ranging from about 2 to about 5 nm. In otherexamples, the capping layer 128 may include Ru compounds such as RuB,RuSi, chromium (Cr), Cr oxide, or Cr nitride.

Referring back to FIG. 1, the operation 12 in the method 10 may furtherinclude other steps, such as alignment after the mask 36 is secured onthe mask stage.

Still referring to FIG. 1, the method 10 also includes an operation 14to load a target 40 to the substrate stage 42 of the lithography system30. In the present embodiment, the target 40 is a semiconductorsubstrate, such as a silicon wafer. The target 40 is coated with aresist layer that is sensitive to the EUV light. The resist layer is tobe patterned by a lithography exposure process such that the IC designlayout (IC pattern) of the mask 36 is transferred to the resist layer.

Referring to FIG. 1, the method 10 includes an operation 16 by settingthe illuminator 34 of the lithography system 30 in a highly coherentillumination mode. The illumination mode is configured such that thefill pupil ratio is less than 20% in one example. In the presentembodiment, an off-axis illumination (OAI) mode is achieved. Referringto FIG. 3, an incident light ray 50, after being reflected from the mask36, is diffracted into various diffraction orders due to presence ofthese mask patterns, such as a 0-th diffraction order ray 51, a −1-stdiffraction order ray 52 and a +1-st diffraction order ray 53. In thedepicted embodiment, the non-diffracted light rays 51 are mostlyremoved. The −1-st and +1-st diffraction order rays, 52 and 53, arecollected by the POB 38 and directed to expose the target 40.

The off-axis illumination mode may be achieved by a mechanism, like anaperture with a certain pattern, such as those illustrated in FIGS.10A-10C, constructed according to various examples. The aperture isconfigured at the illuminator stage to achieve the off-axis illuminationmode. However, the aperture causes the EUV radiation loss.

In the present embodiment, the illuminator 34 includes variousswitchable mirrors or mirrors with other suitable mechanism to tune thereflections of the EUV light from those mirrors. In furtherance of thepresent embodiment, the off-axis illumination mode is achieved byconfiguring the switchable mirrors in the illumination stage such as theEUV light from the radiation source 32 is directed into a pattern (suchas those shown in FIGS. 9A-9C) to achieve the off-axis illumination.

The illumination mode may include different patterns, such as thoseexamples in FIGS. 9A-9C. The illumination pattern is determinedaccording to the IC pattern defined on the mask 36 for the expectedpurpose that includes enhancing the intensity of the EUV light duringthe lithography exposure process.

In FIG. 9A, the illumination mode has an annular pattern 130 where theannular portion 130 is the region being transparent (or in “on” state)to the light from the radiation source 32 and the other portions are in“off” state (blocking). The “on” region means that when the lightreaches the region it will be directed to the mask 36. The “off” regionmeans that when the light reaches the region it will be blocked fromreaching the mask 36. Those terms are also used to describe the pupilfilter. For the present example in FIG. 9A, the EUV light reaching theannular portion 130 will be directed to the mask 36 while the EUV lightreaching the “off” portions will be blocked.

In FIG. 9B, the illumination mode has a quasar pattern 132 where thequasar portions 132 are in “on” state and the rest portions are in “off”state. In other words, the EUV light reaching the quasar portions 132will be directed to the mask 36 while the EUV light reaching the restportions will be blocked.

In FIG. 9C, the illumination mode has a scattering pattern 134. The EUVlight directed to the scattering portions 134 will be directed to themask 36 while the EUV light to the rest portions will be blocked.

Referring to FIG. 1, the method 10 may include an operation 18 byconfiguring a pupil filter 54 in the lithography system 30. The pupilfilter 54 is configured in a pupil plane of the lithography system 30.In an image optical system, there is a plane with field distributioncorresponding to Fourier Transform of object (the mask 36 in the presentcase). This plane is called pupil plane. The pupil filter 54 is placedin the pupil plane to filter out specific spatial frequency componentsof the EUV light directed from the mask 36.

The pattern defined in the pupil filter 54 is determined by theillumination mode. In the present embodiment, the pupil filter 54 isdesigned to filter out the non-diffracted portion of the illuminatedlight directed from the mask 36. In furtherance of the presentembodiment, the pupil filter 54 matches the illumination mode but iscomplimentary. In furtherance of the embodiment, the pattern in thepupil filter 54 is substantially similar to the pattern of theillumination mode. For example, when the illumination mode is defined asthe annular pattern in FIG. 9A, the pattern of the pupil filter 54 isalso the same annular pattern 136 illustrated in FIG. 10A. However, thepattern of the pupil filter in FIG. 10A is complimentary to the patterndefined in the illumination mode in FIG. 9A. Particularly, the annularportion 136 is in the “off” state where the EUV light reaches thisportion in the pupil plane will be blocked. The EUV light reaches toother portion in the pupil plane will be directed to the target 40 (“on”state). Similarly, when the illumination mode is defined in FIG. 9B, thecorresponding pupil filter will have a pattern illustrated in FIG. 10B,wherein the quasar portions 138 are in “off” state while the otherportions are in the “on” state. In another example, when theillumination mode is defined in FIG. 9C, the corresponding pupil filterwill have a pattern illustrated in FIG. 10C, wherein the scatteringportions 140 are in “off” state while the other portions are in the “on”state.

In another embodiment, the pupil filter may have a pattern slightlydifferent from the pattern defined in the illumination mode. Forexample, the pupil filter has an “off” pattern larger than the “on”pattern of the corresponding illumination mode such that thecorresponding “on” region in the illumination mode is covered with anenough margin. Other illumination modes and the corresponding pupilfilters may be used according to other examples.

In yet another embodiment, where the illuminator source is out of pupilor partial coherence sigma is >1, the pupil filter is eliminated. As oneexample illustrated in FIG. 11A, the “on” regions 150 in theillumination mode are illustrated. The full pupil 152 in the pupil planeis illustrated in FIG. 11A in the region within the dashed line forreference. The “on” region 150 in the illumination mode is out of thefull pupil 152. In this case, the sigma center is greater than 1. Inthis particular example, the sigma center is 1.2 and sigma radius is0.05. There is no need to utilize a pupil filter in the pupil plane.Another example is illustrated in FIG. 11B, where the “on” region 154 isout of the full pupil 152. In this case, the sigma center is 1.15greater than 1 and sigma radius is 0.05. As a result, there is no needto utilize the pupil filter in the pupil plane.

Referring back to FIG. 1, the method 10 proceeds to operation 20 byperforming a lithography exposure process to the target 40 in theconfigured illumination mode and the pupil filter (in the cases wherethe pupil filter is needed). The EUV light from the radiation source 32is modulated by the illuminator 34 with the EUV energy distribution forthe off-axis illumination, directed from the mask 36, and furtherfiltered by the pupil filter, the EUV light images the IC pattern of themask 36 to the target with enhanced light.

This is illustrated and described below with reference to FIGS. 12-13and other figures. FIGS. 12 and 13 are diagrammatical view of thespatial distribution of EUV light. The horizontal axis representsspatial dimension and the vertical axis represents the amplitude of theEUV light. In the present embodiment for the illustration, the maskpattern is the IC pattern defined in FIG. 4. The main feature is 80 inthe first mask state and the field 82 is in the second mask state.Accordingly, the EUV light distribution after directed from the mask 36is illustrated in FIG. 12. The light amplitude corresponding to thefirst mask state (the main feature 80) is about 1 (in a relative unitassuming the full amplitude before reaching the mask is 1). This meansthe EUV light associated with the main feature 80 is fully reflectedwithout energy loss and the phase is 0. In contrast, the light amplitudecorresponding to the second mask state (the field 82) is about −1 (inthe relative unit). This means the EUV light associated with the field82 is fully reflected without energy loss and the phase is 180° relativeto that of the main feature.

The EUV light from the mask 36 is further filtered by the pupil filterin the pupil plane such that a portion of the EUV light with a certainspatial frequency is filtered out. In the present embodiment, thenon-diffracted component of the EUV light is filtered out. In oneexample, the EUV component of the 0^(th) spatial frequency is filteredout. The EUV light spatial distribution after the pupil filter isillustrated in FIG. 13. The light amplitude corresponding to the firstmask state (the main feature 80) is about 2 and the light amplitudecorresponding to the second mask state (the field 82) is about 0.Therefore, the amplitude of the EUV light corresponding to the firstmask state is about doubled. Accordingly, the intensity of the EUV lightcorresponding to the first mask state is about four times greater. Thisis achieved by the designed illumination mode and the structure of themask 36 (and additionally contributed by the corresponding pupilfilter). In other embodiment, the first and second mask states mayexperience certain energy loss due to the absorption, and the overallEUV intensity is still substantially enhance, such as about 3 timesgreater than the original EUV intensity.

One real example is further illustrated in FIGS. 14-16. FIG. 14illustrates an IC pattern 160. The IC pattern 160 includes various mainfeatures 162 (three exemplary main features in this example) and thefield 164. By implementing the method 10 with the mask 36, the image ofthe IC pattern on the target 40, illustrated in FIG. 15, is achievedwith high intensity. In this case, the IC pattern is defined on the BPM36. In the present embodiment, the main features 162 are defined in oneof the first and second mask states. The field 164 is defined in anothermask state.

As a comparison, when the IC pattern is defined in a conventional mask,such as a binary intensity mask, the corresponding image of the ICpattern on the target, as illustrated in FIG. 16, has a low intensity.Other benefits of the method 10 includes reduced mask error enhancementfactor (MEEF) and reduced printability of particles on the mask. TheMEEF reduction is further described according to different examples.

FIG. 17 illustrates diagrammatically the MEEF for various methods. MEEFis defined as M*(ΔCD_(w))/(ΔCD_(m)), where ΔCD_(w) is the CD change of afeature in wafer and ΔCD_(m) is the CD change of the feature in themask. The horizontal axis represents dimension on mask (DOM) innanometer (nm). The vertical axis represents MEEF. The dimension onwafer (DOW) is about 18 nm in the present example. FIG. 17 includes fourcurves. The first curve represents the data from the lithographyexposure process using the mask 36 (BPM) with photoresist diffusionlength (DL)=0 nm corresponding to ideal resist, labeled in the legend as“BPM-DL=0”. The second curve represents the data from the lithographyexposure process using a binary intensity mask with DL=0 nm, labeled inthe legend as “BIM-DL=0”. The third curve represents the data from thelithography exposure process using the mask 36 with DL=6 nm, labeled inthe legend as “BPM-DL=6”. The fourth curve represents the data from thelithography exposure process using a binary intensity mask with DL=6 nm,labeled in the legend as “BIM-DL=6”. FIG. 17 clearly demonstrates thatthe MEEF is substantially reduced by utilizing the method 10 with themask 36.

FIG. 18 illustrates the mask 36 that is the same mask illustrated inFIG. 5A. However, there is an exemplary particle 166 falling on the mask36 in FIG. 18. According to the similar analysis in FIGS. 12 and 13, theEUV light distribution in amplitude before the pupil filter is similarto the one in FIG. 12 but the region corresponding to the particlecompletely lost the EUV light or the corresponding amplitude is 0. Afterthe pupil filter, the EUV light distribution in amplitude is similar tothe one in FIG. 13 but the region corresponding to the particle has theamplitude as 1. Accordingly, the EUV intensity to the field is 0, to themain feature 4 and to the particle is 1. The relative EUV intensity tothe particle 166 is non-zero, which is different from the intensity tothe field. The printability of the particle is reduced.

In contrast, the particle falling on the main feature in a binaryintensity mask will cause the total loss of the EUV light reachingthereto, resulting in an unexposed region as a defect.

Referring back FIG. 1, the method 10 may further include otheroperations. For example, the method 10 includes an operation 22 bydeveloping the exposed resist layer coated on the target 40, therebyforming a patterned resist layer with one or more openings imaged fromthe IC pattern defined on the mask 36.

In another example, the method 10 further includes an operation 24 byperforming a fabrication process to the target 40 through the patternedresist layer. In one embodiment, the substrate or a material layer ofthe target is etched through the openings of the patterned resist layer,thereby transferring the IC pattern to the substrate or the underlyingmaterial layer. In furtherance of the embodiment, the underlyingmaterial layer is an interlayer dielectric (ILD) layer disposed on thesemiconductor substrate. The etching process will form contacts or viasin the corresponding ILD layer. In another embodiment, an ionimplantation process is applied to the semiconductor substrate throughthe openings of the patterned resist layer, thereby forming dopedfeatures in the semiconductor substrate according to the IC pattern. Inthis case, the patterned resists layer functions as an ion implantationmask.

Various embodiments of the method 10 and the mask 36 are describedaccording to the present disclosure. Other alternatives andmodifications may present without departure from the spirit of thepresent disclosure. In one embodiment, the IC pattern defined on themask 36 may further include various assist polygons incorporated by anOPC process. In one example, the assist polygons are assigned to a samestate. For example, the assist polygons are assigned to the first maskstate. In another embodiment, the binary phase mask 36 may have otherstructure to achieve the same functions, such as enhancing the exposureintensity by the method 10. In various examples, the resist material isassumed as a positive tone resist and the main features achieve the highexposure intensity. However, in one embodiment, the resist layer may bea negative tone resist.

As described above in various embodiments, the present disclosureprovides a method for extreme ultraviolet lithography (EUVL) exposureprocess to pattern an IC pattern, especially an IC pattern with a lowpattern density, with enhanced intensity by using a binary phase mask,off-axis illumination mode and corresponding pupil filter. Especially,the illumination mode is determined by the IC pattern defined on thebinary phase mask and the pattern of the pupil filter is determinedaccording to the illumination mode. In one embodiment, the illuminatorincludes a plurality of mirrors configured to generate the illuminationmode. The pupil filter is configured in the pupil plane of thelithography system and is designed to filter out a portion of the EUVlight with a certain spatial frequency. In the present example, thenon-diffracted component of the EUV light is filtered out. In anotherembodiment, the pupil filter may be eliminated during the lithographyexposure process when the sigma center in the illumination mode isgreater than 1.

Various advantages may present in different embodiments of the presentdisclosure. In one example, the exposure intensity is enhanced.Accordingly, the exposure duration is reduced and the throughput isincreased, especially for the IC pattern with a low pattern density. Byutilizing the highly coherent illumination mode and corresponding pupilfiltering, the energy loss is substantially reduced. In one example forillustration, the pupil fill ratio (the relative energy loss by thepupil filter) is much less, such as less than about 20%. The exposurelight amplitude to the main features is substantially increased and theMEEF is reduced. In another example, the printability of the fallingparticles is mitigated.

Thus, the present disclosure provides a method for extreme ultravioletlithography (EUVL) process in some embodiments. The method includesloading a binary phase mask (BPM) to a lithography system, wherein theBPM includes two phase states and defines an integrated circuit (IC)pattern thereon; setting an illuminator of the lithography system in anillumination mode according to the IC pattern; configuring a pupilfilter in the lithography system according to the illumination mode; andperforming a lithography exposure process to a target with the BPM andthe pupil filter by the lithography system in the illumination mode.

The present disclosure also provides a method for EUVL process in otherembodiments. The method includes loading a binary phase mask (BPM) to alithography system, wherein the BPM includes two phase states anddefines an integrated circuit (IC) pattern thereon; setting anilluminator of the lithography system in a highly coherent illuminationmode according to the IC pattern; and performing a lithography exposureprocess to a resist layer coated on a target with the BPM and theilluminator in the illumination mode.

The present disclosure also provides a method for EUVL process in one ormore embodiments. The method includes loading a binary phase mask (BPM)to a lithography system, wherein the BPM includes two phase states anddefines an integrated circuit (IC) pattern with a pattern density lessthan 25%; setting switchable mirrors in an illuminator of thelithography system in an illumination mode; configuring a pupil filterin a pupil plane of the lithography system, wherein the pupil filter hasa pattern determined according to the illumination mode; and performinga lithography exposure process to a target with the BPM and the pupilfilter by the lithography system in the coherent illumination mode.

Another embodiment of the present disclosure is described below withreference to FIGS. 19-22.

Referring to FIG. 19, an EUVL process 210 that may benefit from one ormore embodiments of the present invention is disclosed. The EUVL process210 employs an EUV source 220 that emits radiation having a wavelength λof about 1-100 nm, including an EUV wavelength of about 13.5 nm.

The EUVL process 210 also employs an illuminator 230. The illuminator230 may comprise refractive optics, such as a single lens or a lenssystem having multiple lenses (zone plates) and/or reflective optics,such as a single mirror or a mirror system having multiple mirrors inorder to direct light from the radiation source 220 onto a mask 240. Inthe EUV wavelength range, reflective optics is employed generally.Refractive optics, however, can also be realized by zoneplates. In thepresent embodiment, the illuminator 230 is set up to provide a nearlyon-axis illumination to illuminate the mask 240. In nearly on-axisillumination, all incoming light rays incident on the mask are at thesame angle of incidence (AOI), e.g., AOI=6°, as that of a chief ray. Inmany situations, there may be some angular spread of the incident light.For example, the EUVL process 210 may utilize disk illumination (i.e.,the shape of the illumination on the pupil plane is like a disk centeredat the pupil center). When illumination of a partial coherence σ, e.g.,σ=0.3, is employed, the maximum angular deviation from the chief ray issin⁻¹[m×σ×NA], where m and NA are the magnification and numericalaperture, respectively, of the projection optics box (POB) 250 to bedetailed below. Partial coherence 6 can also be used to describe a pointsource which produces a plane wave for illuminating the mask 240. Inthis case, the distance from the pupil center to the point source in thepupil plane is NA×σ and the AOI of the corresponding plane wave incidenton the mask 240 is sin⁻¹[m×σ×NA]. In the present embodiment, it issufficient to employ a nearly on-axis illumination consisting of pointsources with σ less than 0.3.

The EUVL process 210 also employs a mask 240 (in the present disclosure,the terms mask, photomask, and reticle are used to refer to the sameitem). The mask 240 can be a transmissive mask or a reflective mask. Inthe present embodiment, the mask 240 is a reflective mask such asdescribed in further detail below. The mask 240 may incorporate otherresolution enhancement techniques such as phase-shifting mask (PSM)and/or optical proximity correction (OPC).

The EUVL process 210 also employs a projection optics box (POB) 250. ThePOB 250 may have refractive optics or reflective optics. The radiationreflected from the mask 240 (e.g., a patterned radiation) is collectedby the POB 250. The POB 250 also includes a pupil filter placed at anoptics pupil plane to modulate phase and amplitude of radiationreflected from the mask 240.

Referring to FIG. 20, an incident light ray 260, after being reflectedfrom the mask 240, is diffracted into various diffraction orders due tothe presence of mask patterns, such as a 0-th diffraction order ray 261,a −1-st diffraction order ray 262 and a +1-st diffraction order ray 263and other higher diffraction order rays (represented by 264 and 265).For lithographic imaging, purely coherent illumination is generally notemployed. Disk illumination with partial coherence σ being at most 0.3generated by the illuminator 230 is employed. In the depictedembodiment, the non-diffracted light rays 261 are mostly (e.g., morethan 70%) removed by the pupil filter in the POB 250. The −1-st and+1-st diffraction order rays, 262 and 263, and other higher diffractionorder rays (264 and 265) are collected by the POB 250 and directed toexpose a target 270. Removing the non-diffracted light amounts tosubtracting the average electric field from the total electric field onthe target 270. For a mask with a layout of low pattern density, theaverage electric field is close to the reflection coefficient of thefield of the mask, i.e., the region without polygons. Therefore, for amask with a layout of low pattern density, removing the non-diffractedlight greatly enhances the image log slope of the aerial image formed onthe target 270, since the phase difference of electric fields on patternregions and on the field region is close to 180 degrees.

The target 270 includes a semiconductor wafer with a photosensitivelayer (e.g., photoresist or resist), which is sensitive to the EUVradiation. The target 270 may be held by a target substrate stage. Thetarget substrate stage provides control of the target substrate positionsuch that the image of the mask is scanned onto the target substrate ina repetitive fashion (though other lithography methods are possible).

The following description refers to the mask 240 and a mask fabricationprocess. The mask fabrication process includes two steps: a mask blankfabrication process and a mask patterning process. During the mask blankfabrication process, a mask blank is formed by depositing suitablelayers on a suitable substrate. The mask blank is patterned during themask patterning process to have a design of a layer of an integratedcircuit (IC) device (or chip). The patterned mask is then used totransfer circuit patterns (e.g., the design of a layer of an IC device)onto a semiconductor wafer. The patterns can be transferred over andover onto multiple wafers through various lithography processes. Severalmasks (for example, a set of 15 to 30 masks) may be used to construct acomplete IC device.

Referring to FIG. 21, a mask blank 300 comprises a substrate 310 made oflow thermal expansion material (LTEM). The LTEM material may includeTiO₂ doped SiO₂, and/or other low thermal expansion materials known inthe art. The LTEM substrate 310 serves to minimize image distortion dueto mask heating. In the present embodiment, the LTEM substrate includesmaterials with a low defect level and a smooth surface. In addition, aconductive layer 305 may be deposed under (as shown in the figure) theLTEM substrate 310 for the electrostatic chucking purpose. In anembodiment, the conductive layer 305 includes chromium nitride (CrN),though other compositions are possible.

The mask blank 300 includes a reflective multilayer (ML) 320 depositedover the LTEM substrate 310. According to Fresnel equations, an incidentlight ray will be partially reflected when it propagates across theinterface between two materials of different refractive indices. Thereflected light ray is larger when the difference of the refractiveindices is larger. To increase the reflected light ray, one may alsoincrease the number of interfaces by depositing a ML of alternatingmaterials, and then choose an appropriate thickness for each layer ofthe ML according to the wavelength and the angle of incidence of theincident light ray so that reflected light rays from differentinterfaces interfere constructively. However, the absorption of theemployed materials for the ML limits the highest reflectivity that canbe achieved. In one embodiment, the reflective ML 320 includesmolybdenum-silicon (Mo/Si) film pairs (i.e., a layer of molybdenum overa layer of silicon in each film pair). In another embodiment, thereflective ML 320 includes molybdenum-beryllium (Mo/Be) film pairs. Instill another embodiment, the reflective ML 320 includes forty Mo/Sifilm pairs with each Mo/Si film pair consisting of 3-nm Mo and 4-nm Si.In this case, a reflectivity of about 70% is achieved.

The mask blank 300 may also include a capping layer 330 over thereflective ML 320 to prevent oxidation of the reflective ML 320. In oneembodiment, the capping layer 330 includes silicon with about 4-7 nmthickness.

The mask blank 300 may also include a buffer layer 340 over the cappinglayer 330 to act as an etching stop layer in a patterning or repairingprocess of an absorption layer, which will be described later. Thebuffer layer 340 has different etching characteristics from theabsorption layer. The buffer layer 340 includes ruthenium (Ru), Rucompounds such as RuB, RuSi, chromium (Cr), Cr oxide, and Cr nitride. Alow temperature deposition process is often chosen for the buffer layerto prevent inter-diffusion of the reflective ML 320. In the presentembodiment, the buffer layer 340 includes ruthenium with a thicknessfrom 2 nm to 5 nm. In one embodiment, the capping layer and the bufferlayer are a single layer.

In the present embodiment, the mask blank 300 includes a phase-shiftinglayer 350 over the buffer layer 340. The phase-shifting layer 350includes material or materials whose thickness or thicknesses is or areproperly chosen to achieve an about 180-degree phase shift for a lightray reflected from this region (relative to the region without thephase-shifting layer). In one embodiment, the phase-shifting layer 350includes molybdenum (Mo) having a thickness from 40 nm to 48 nm. Thephase-shifting layer 350 may also be formed by multiple layers ofdifferent materials.

One or more of the layers 305, 320, 330, 340 and 350 may be formed byvarious methods, including physical vapor deposition (PVD) process suchas evaporation and DC magnetron sputtering, a plating process such aselectrode-less plating or electroplating, a chemical vapor deposition(CVD) process such as atmospheric pressure CVD (APCVD), low pressure CVD(LPCVD), plasma enhanced CVD (PECVD), or high density plasma CVD (HDPCVD), ion beam deposition, spin-on coating, metal-organic decomposition(MOD), and/or other methods known in the art. The MOD is a depositiontechnique by using a liquid-based method in a non-vacuum environment. Byusing MOD, a metal-organic precursor, dissolved in a solvent, isspin-coated onto a substrate and the solvent is evaporated. A vacuumultraviolet (VUV) source is used to convert the metal-organic precursorsto constituent metal elements.

Referring to FIG. 22, the phase-shifting layer 350 is patterned to formthe design layout pattern EUV mask 400 having first and second regions,410 and 420. The phase-shifting layer 350 is patterned by removingmaterial from the second region 420 while the material remains in thefirst region 410. The patterned phase-shifting layer produces an about180-degree phase shift (for the reflected light ray from the firstregion 410 with respect to the reflected light ray from the secondregion 420). In the depicted embodiment, in the EUV mask 400 consistingof a layout of low pattern density, the distance between two polygons inthe layout is not smaller than about λ/NA. As an example, the lowpattern density layer mask 400 is a via layer mask and the second region420 represent via patterns. Actually, according to the explanationstated above, either the first region 410 or the second region 420 canbe used to define via patterns.

The phase-shifting layer 350 can be patterned by various patterningtechniques. One such technique includes using a resist coating (e.g.,spin-on coating), soft baking, mask aligning, exposure, post-exposurebaking, developing the resist, rinsing, and drying (e.g., hard baking).An etching process is followed to remove portions of the phase-shiftinglayer 350 and form the first region 410. The etching process may includedry (plasma) etching, wet etching, and/or other etching methods. Forexample, the dry etching process may implement a fluorine-containing gas(e.g., CF4, SF6, CH2F2, CHF3, and/or C2F6), chlorine-containing gas(e.g., C12, CHC13, CC14, and/or BC13), bromine-containing gas (e.g., HBrand/or CHBR3), iodine-containing gas, other suitable gases and/orplasmas, and/or combinations thereof. Alternative patterning processesinclude maskless photolithography, electron-beam writing,direct-writing, and/or ion-beam writing.

Based on the above, the present disclosure offers the EUVL process 210employing a nearly on-axis illumination, e.g., a disk illumination withpartial coherence σ smaller than 0.3 to expose a mask to producediffracted light and non-diffracted light. The EUVL process 210 employsa pupil filter to removes more than 70% of the non-diffracted light soas to obtain the benefit of throughput enhancement. The EUVL process 210also employs a mask with two regions formed by a patternedphase-shifting layer over the ML. The EUVL process 210 demonstrates anenhancement of aerial image contrast and throughput improvement of lowpattern density layer.

The present disclosure is directed towards lithography systems andprocesses. In one embodiment, an extreme ultraviolet lithography (EUVL)process comprises receiving a mask. The mask includes a low thermalexpansion material (LTEM) substrate, a reflective multilayer (ML) overone surface of the LTEM substrate, a first region having aphase-shifting layer over the reflective ML, and a second region havingno phase-shifting layer over the reflective ML. The EUVL process alsocomprises exposing the mask by a nearly on-axis illumination withpartial coherence σ less than 0.3 to produce diffracted light andnon-diffracted light, removing at least a portion of the non-diffractedlight, and collecting and directing the diffracted light and the notremoved non-diffracted light by a projection optics box (POB) to exposea target.

In another embodiment, an extreme ultraviolet lithography (EUVL) processcomprises receiving a mask. The mask has a first region and a secondregion. The phase difference between the first region and the secondregion is about 180 degrees and the reflectivity of the first region ismore than 20% of the reflectivity of the second region. The EUVL processalso comprises exposing the mask by a nearly on-axis illumination withpartial coherence σ less than 0.3 to produce diffracted light andnon-diffracted light, removing more than 70% of the non-diffractedlight, and collecting and directing the diffracted light and the notremoved non-diffracted light by a projection optics box (POB) to exposea semiconductor wafer.

The present disclosure is also directed towards masks. In still anotherembodiment, a mask for extreme ultraviolet lithography (EUVL) comprisesa low thermal expansion material (LTEM) substrate, a reflectivemultilayer (ML)over one surface of the LTEM substrate, a conductivelayer above an opposite surface of the LTEM substrate, a patternedphase-shifting layer over the reflective ML to define a first region anda second region. The phase difference between the first region and thesecond region is about 180 degrees and the reflectivity of the regionwith the phase-shifting layer is more than 20% of the reflectivity ofthe region without the phase-shifting layer.

Another embodiment of the present disclosure is described below withreference to FIGS. 23-31.

FIGS. 23A, 23B, and 23C illustrate planar views of different examples ofan illuminator 500. The illuminator 500 is similar to the illuminatordiscussed above with reference to FIGS. 9A-9B. For example, theilluminator 500 includes various switchable mirrors (or mirrors withother suitable mechanism) to tune the reflections of the EUV light(generated from the radiation source 32, as discussed above in FIG. 2)from those mirrors. Similar to the illuminator discussed above, theilluminator 500 can achieve an off-axis illumination mode.

As is shown in FIGS. 23A-23C, the illuminator 500 may have differentpatterns. In FIG. 23A, the illuminator 500 has an annular pattern 530(also referred to as annular portion). The annular portion 530 is theregion being transparent (or in “on” state) to the EUV light from theradiation source 32. The other portions (the rest of the illuminator500) are in “off” state (blocking). The “on” region means that when theEUV light reaches the region, it will be directed to the mask 36 of FIG.2. The “off” region means that when the EUV light reaches the region, itwill be blocked from reaching the mask 36. Thus, for the example in FIG.23A, the EUV light reaching the annular portion 530 will be directed tothe mask 36, while the EUV light reaching the “off” portions will beblocked.

In FIG. 23B, the illuminator 500 has a quasar pattern 532. The quasarpattern 532 includes a plurality of quasar portions 532 (four in thisexample) distributed similar to the four corners of a square. The quasarportions 532 are in “on” state, and the remaining portions of theilluminator are in “off” state. In other words, the EUV light reachingthe quasar portions 532 will be directed to the mask 36, while the EUVlight reaching the remaining portions will be blocked.

In FIG. 23C, the illuminator 500 has a scattering pattern 534, whichinclude a plurality of “on” portions 534 scattered throughout theilluminator 500. The EUV light directed to the scattering portions 534will be directed to the mask 36 while the EUV light to the remainingportions of the illuminator 500 will be blocked.

FIGS. 24A, 24B, and 24C illustrate planar views of different examples ofa pupil phase modulator 600 according to embodiments of the presentdisclosure. Similar to the pupil filter 54 discussed above, the pupilphase modulator 600 is positioned in a pupil plane of the lithographysystem 30, which is a plane with a field distribution corresponding to aFourier Transform of the mask 36 in the present disclosure. Unlike thepupil filter, however, the pupil phase modulator is not configured tofilter out specific spatial frequency components of the EUV lightdirected from the mask 36, but rather to change or shift a phase of the0-th order diffracted ray discussed above. For example, in someembodiments, the pupil phase modulator 600 is designed such that a 0-thorder diffracted ray (such as the 0-th order ray 51 of FIG. 3) isphase-inverted by 180 degrees.

This phase-inversion of the 0-th order diffracted ray is illustrated inFIGS. 25A and 25B. In more detail, FIG. 25A is a diagrammatic view of aspatial distribution of EUV light before phase-inverting the 0-th orderdiffracted light, and FIG. 25B is a diagrammatic view of a spatialdistribution of EUV light after phase-inverting the 0-th orderdiffracted light. The horizontal axis represents spatial dimension, andthe vertical axis represents the amplitude of the EUV light. Thehorizontal axis intersects the vertical axis at a zero amplitude of theEUV light.

The 0-th order diffracted light ray and the higher order diffractedlight rays are illustrated separately in both FIG. 25A and FIG. 25B. Inmore detail, FIG. 25A illustrates a 0-th order diffracted light ray 602Awithout phase inversion, and the ray 602A has a negative amplitude 605A.FIG. 25B illustrates the higher order diffracted light rays withoutphase inversion, and they have a negative floor amplitude 615. In otherwords, while the light amplitude corresponding to the first mask state(e.g., the main feature 80) discussed above has an amplitude that ismostly positive, the light amplitude corresponding to the second maskstate (e.g., the field 82) discussed above may have a negative amplitude615. This negative amplitude may be referred to as the floor amplitude,which is negative in this example.

In the embodiment illustrated herein, the magnitude or absolute value ofthe amplitude 605A is greater than the amplitude 615. That is, theamplitude 605A of the 0-th order diffracted light ray 602A is “morenegative” than the floor amplitude 615 of the higher order diffractedlight rays 610. The 0-th order diffracted light ray 602A and the highorder diffracted light rays 610 combine to produce a combined light620A, which also has a negative floor amplitude 625A, since both theamplitude 605A of the diffracted 0-th order light ray 602A and theamplitude 615 of the higher order diffracted light rays are negative.

In comparison, the 0-th order diffracted light ray 602B in FIG. 25B hasundergone phase inversion, for example a phase inversion of 180 degrees.Consequently, the phase-inverted light ray 602B has a positive amplitude605B. As discussed above, the phase inversion of the 0-th orderdiffracted light 602A is accomplished by the pupil phase modulator 600of the present disclosure.

The pupil phase modulator 600 is also specifically configured toattenuate the magnitude of the amplitude 605A of the 0-th orderdiffracted light ray 602A by a predetermined ratio. As such, thephase-inverted light ray 602B has a smaller magnitude (or absolute)value for its amplitude 605B than the amplitude 605A of the non-invertedlight ray 602A, even though the amplitude 605B is positive, and theamplitude 605A is negative. The amplitude attenuation herein isperformed so that the phase-inverted light 0-th order ray 602B, afterbeing combined with the higher order rays 610 (which is the same in bothFIGS. 25A and 25B, since the pupil phase modulator 600 does not phaseinvert the higher order diffracted light), will produce a combined light620B whose floor amplitude 625B is substantially zero. In other words,the pupil phase modulator 600 attenuates, and inverts, the amplitude ofthe 0-th order diffracted light ray so that the phase-inverted amplitude(now positive) cancels out the negative amplitude 615 of the higherorder diffracted light rays 610. The end result—the light 620B—is animprovement over the light 620A that would have been produced withoutthe amplitude attenuation and the phase-inversion by the pupil phasemodulator 600 herein. Among other things, the light 620B offers improvedthroughput.

The pupil phase modulator 600 achieves the phase-inversion and theamplitude attenuation discussed above by carefully configuring its shapeand material compositions. Referring back to FIGS. 24A-24C, the pupilphase modulator 600 has a planar shape that corresponds or matches theilluminator 500. In other words, the pattern in the pupil phasemodulator 600 is substantially similar to the pattern of thecorresponding illumination mode. For example, when the illumination modeis defined as the annular pattern in FIG. 23A, the pattern of the pupilphase modulator 600 is also the same annular pattern 636 illustrated inFIG. 24A. However, the pattern of the pupil phase modulator 600 in FIG.24A is complimentary to the pattern defined in the illumination mode inFIG. 23A. Particularly, the annular portion 636 is configured to providethe phase inversion and amplitude attenuation for the 0-th orderdiffracted ray discussed above. The remaining portions of the pupilphase modulator 600 may be transparent, for example as a void or avacuum, and as such they may be referred to be in an “on” state. Thus,EUV light reaching the portions of the pupil phase modulator 600 otherthan the annular pattern 636 will be directed to the target wafer.

Similarly, when the illumination mode is defined in FIG. 23B, thecorresponding pupil phase modulator 600 will have a pattern illustratedin FIG. 24B, wherein the quasar portions 638 are configured to providethe phase inversion and amplitude attenuation for the 0-th orderdiffracted ray, while the other portions are in the “on” state (e.g.,being a void). In another example, when the illumination mode is definedin FIG. 23C, the corresponding pupil phase modulator 600 will have apattern illustrated in FIG. 24C, wherein the scattering portions 640 areconfigured to provide the phase inversion and amplitude attenuation forthe 0-th order diffracted ray, while the other portions are in the “on”state (e.g., being a void or vacuum).

It is understood that the patterns 636, 638 and 640 of the pupil phasemodulator 600 need not be an exact match of the corresponding patterns530, 532, and 534 of the illuminator 500. For example, in someembodiments, the patterns 636, 638 and 640 may be slightly larger thanthe corresponding patterns 530, 532, and 534 of the illuminator 500, soas to provide a margin to cover the patterns 530, 532, and 534.

In the embodiments illustrated in FIGS. 24A-24C, the pupil phasemodulator 600 is a device that may be switched in or out of the EUVlithography system depending on the illuminator type. For example, if anannular illumination mode is needed, the pupil phase modulator 600 ofFIG. 24A may be loaded into the EUV lithography system. If a quasarillumination mode is needed, the pupil phase modulator 600 of FIG. 24Amay then be replaced with the pupil phase modulator 600 of FIG. 24B.

It is also understood that, as a practical matter, support structuresmay be implemented to connect or support the patterns 636, 638, or 640.For example, referring to FIGS. 26A-26B, the pupil phase modulator 600with the annular pattern 636 and the pupil phase modulator 600 with thequasar pattern 638 are illustrated, respectively. Each pupil phasemodulator 600 may have an outer ring or rim, which may define an outerboundary 650 of the pupil phase modulator 600. The annular pattern 636or the quasar pattern 638 are implemented with one or morephase-shifting layers (and supporting layers), which will be discussedbelow in more detail. The patterns 636 and 638 provide the phaseshifting and the amplitude attenuation for the 0-th order ray.

However, the rest of the pupil phase modulator 600, for example regions660, are in the “ON” state and are transparent. In some embodiments,these regions 660 may be a void or a vacuum. Consequently, a mechanicalstructure is needed to provide support or connection between thepatterns 636/638 and the boundary 650. In the embodiment shown in FIGS.26A-26B, rods 670 may be implemented to connect the annular pattern 636to the boundary 650, or to connect the quasar pattern 638 to theboundary 650. The number of the rods 670 and the locations of the rods670 may vary from embodiment to embodiment, depending on the patternshape and other considerations such as durability, etc.

The implementation details of the phase-shifting patterns (e.g., theannular pattern 636 or the quasar pattern 638) are now discussed in moredetail with reference to FIG. 27. Specifically, FIG. 27 illustrates adiagrammatic cross-sectional side view of the annular pattern 636 as anexample. It is understood that the quasar pattern 638 or the scatteringpattern 640 may also be implemented similarly.

The pattern 636 includes a mechanical support layer 700. Aphase-shifting layer 710 is disposed over the mechanical support layer700. Another mechanical support layer 720 is disposed over thephase-shifting layer 710. In some embodiments, the mechanical supportlayers 700 and 720 each contain silicon. Silicon is selected because itis nearly transparent in the EUV range, and it also has good mechanicalproperties for serving as mechanical support. In some embodiments, athickness of each of the mechanical support layers 700 and 720 isconfigured to be within a range from 50 nanometers (nm) to 3 microns.

The phase-shifting layer 710 may contain a material such as molybdenum,ruthenium, zirconium, nickel, or combinations thereof. These materialsare configured to cause a phase shift of about 180 degrees for the 0-thorder diffracted light ray. As discussed above with reference to FIGS.25A-25B, this phase shift inverts the amplitude of the 0-th orderdiffracted ray from a negative amplitude to a positive amplitude. Athickness of the phase-shifting layer 710 is also configured toattenuate the magnitude of the amplitude of the 0-th order ray. Asdiscussed above with reference to FIGS. 25A-25B, the amplitudeattenuation is configured such that the attenuated and phase-invertedamplitude would cancel out the negative floor amplitude of the higherorder diffracted rays. Since the amount of the amplitude attenuationdepends at least in part on the thickness of the layer 710, thethickness of the layer 710 is carefully configured. For example, agreater thickness may be associated with a greater amount of amplitudeattenuation. In some embodiments, the thickness of the layer 710 is in arange from 20 nm to 200 nm.

In some alternative embodiments, the pattern 636 may be implemented withmore than one phase-shifting layer, so as to provide additional degreesof freedom in tuning the phase shifting and/or amplitude attenuation.For example, FIGS. 28A, 28B, and 28C each illustrate a diagrammaticcross-sectional side view of an embodiment of the annular pattern 636that contains more than one phase-shifting layer. Referring to FIG.28A-28B, the pattern 636 includes the mechanical support layers 700 and720, as well as phase-shifting layers 710 and 730. In FIG. 28A, thephase shifting layer 730 is disposed between the phase-shifting layer710 and the mechanical support layer 720, whereas in FIG. 28B, the phaseshifting layer 730 is disposed between the phase-shifting layer 710 andthe mechanical support layer 700. The phase-shifting layers 710 and 730have different material compositions. For example, in some embodiments,the phase-shifting layer 710 contains molybdenum, and the phase-shiftinglayer 730 contains zirconium or nickel. In some embodiments, thethickness for the phase-shifting layer 730 may be in a range from 2 nmto 50 nm.

In the embodiment shown in FIG. 28C, the pattern 636 includes threedifferent phase-shifting layers 710, 730 and 740. The phase-shiftinglayer 710 is “sandwiched” between the phase-shifting layers 730 and 740,which are collectively “sandwiched” between the mechanical supportlayers 700 and 720. In some embodiments, the phase-shifting layer 710contains molybdenum, the phase-shifting layer 730 contains zirconium,and the phase-shifting layer 740 contains nickel. In another embodiment,the phase-shifting layer 710 contains molybdenum, the phase-shiftinglayer 730 contains nickel, and the phase-shifting layer 740 containszirconium.

Having more than one phase-shifting layer helps provide additionalfreedom in attenuating the amplitude of the 0-th order diffracted light.This is at least in part due to the different material compositions ofthe phase-shifting layers 710, 730, or 740. Using the embodiment shownin FIG. 28C a simplified example, suppose that the phase-shifting layer710 contains molybdenum, which may attenuate the amplitude by 70%, andthe phase-shifting layer 730 contains zirconium, which may attenuate theamplitude by 50%, and that the phase-shifting layer 740 contains nickel,which may attenuate the amplitude by 60%. Given these parameters, therespective thicknesses of these layers 710, 730, and 740 may beconfigured using various linear combinations to derive a desiredamplitude attenuation percentage, for example 65%. In some embodiments,the thickness of the phase-shifting layer 710 is in a range from 20 nmto 200 nm, and the thickness of the phase-shifting layers 730 and 740are each in a range from 2 nm to 50 nm.

In some embodiments, the thickness of the layer 730 or 740 may beoptionally further configured to reduce unwanted reflection. Forexample, the thickness of the layer 730 (or 740) may be set to be equalto (n+0.5)*(λ/2), where n is an integer in a range from 0 to 30 (forexample from 0 to 20), and λ is the wavelength of the EUV light (e.g.,13.5 nm). Setting the thickness of the layers 730 (and/or 740) accordingto the equation (n+0.5)*(λ/2) helps minimize the destructiveinterference at the layer interfaces, thereby minimizing unwantedreflection. After setting the thickness of the phase-shifting layer 730(and 740 if 740 is also implemented), the thickness of thephase-shifting layer 710 may then be calculated, so that thephase-shifting layers 710 and 730 (and 740 if it is implemented)collectively may attenuate the 0-th order diffraction light ray by apredetermined percentage or ratio.

It is understood that the materials (molybdenum, zirconium, nickel, orruthenium) of the phase-shifting layers 710, 730, and 740 discussedherein are optimized for a 13.5 nm EUV light. However, the concepts ofthe present disclosure also applies to other EUV light wavelengths,though the material compositions and/or the thicknesses of thephase-shifting layers may need to be reconfigured to optimize thephase-inversion and the amplitude attenuation for the 0-th orderdiffraction ray if another EUV light with a different wavelength isused.

It is also understood that the attenuation percentage or ratio of the0-th order diffraction light may change somewhat depending on theillumination mode. For example, the annular type illuminator maycorrespond to a first desired amplitude attenuation percentage, thequasar type illuminator may correspond to a second desired amplitudeattenuation percentage, and the scattering type illuminator maycorrespond to a third desired amplitude attenuation percentage, wherethe first, second, and third percentages are different from one another.As such, the present disclosure may customize the thicknesses of thephase-shifting layer(s) in the pupil phase modulator 600 to account foreach type of illuminator.

Again, it is understood that each of the patterns 636, 638, and 640discussed above may be implemented using the structures illustrated inFIGS. 27 and 28A-28C that contain either a single phase-shifting layeror multiple phase-shifting layers. Most of the remaining regions of thepupil phase modulator 600—e.g., the regions 660 shown in FIGS.26A-26B—may be implemented as a void or a vacuum. However, as also shownin FIGS. 26A-26B, mechanical support structures such as the rods 670 maybe used to establish a mechanical connection between the patterns636/638 to the boundary 650 of the pupil phase modulator 600. Thisapproach is certainly feasible and offers the advantages such asincreased throughput. However, a potential issue may arise due to thepresence of the mechanical support structures such as the rods 670. Evenif the rods 670 are implemented as silicon (which is closed to beingtransparent in the EUV spectrum), there may still be lack of uniformitydue to the difference between the rods 670 and the regions 660, whichare voids or vacuum. This may have potentially adverse effects on theimage quality.

Therefore, if image quality is an important concern, the pupil phasemodulator 600 may also be implemented according to an alternativeembodiment shown in FIGS. 29A-29B. In more detail, FIG. 29A is asimplified diagrammatic planar view the pupil phase modulator 600 usingthe annular pattern 636 as an example, and FIG. 29B is a simplifieddiagrammatic cross-sectional side of the pupil phase modulator 600 ofFIG. 29A, where the cross-section is taken from point A to point A′. Theannular pattern 636 is implemented in a manner similar to the embodimentdiscussed above with reference to FIG. 27, in that it containsmechanical support layers 700 and 720 and a phase-shifting layer 710disposed in between. However, the embodiment shown in FIG. 29B has anextended mechanical support layer 700 that extends laterally beyond theplanar view boundaries of the annular pattern 636. This extendedmechanical support layer 700 may also be referred to as a siliconmembrane 700.

As is shown in the planar view of FIG. 29A, this embodiment of the pupilphase modulator 600 no longer needs the mechanical support structures670, because the mechanical support layer 700 of the annular pattern 636extends laterally throughout the pupil phase modulator 600 to providethe needed mechanical support. In this manner, the annular pattern 636does not need to be connected to the rod 670 in order to avoid being a“free-floating” structure, since the annular pattern 636 (containing thelayers 710-720) is “sitting” on the mechanical support layer 700 thatnow extends throughout the pupil phase modulator 600. By doing so, thepupil phase modulator 600 in FIGS. 29A-29B has improved uniformity,since the regions 660 is now comprised of the mechanical support layer700, compared to the previous embodiment where the regions 660 arecomprised of both the rods 670 and void or vacuum.

To minimize the light absorption, the mechanical support layer 700 maybe implemented as a silicon layer. However, the presence of themechanical support layer 700 (even if it's implemented with silicon,which is close to being transparent) may still reduce throughput alittle bit compared to the embodiment discussed above with reference toFIGS. 26A-26B. As such, a tradeoff may be made in each case betweenimage quality and throughput, where the pupil phase modulator 600 may beimplemented using the embodiment shown in FIGS. 26A-26B (with theconnection rods 670) if throughput is a greater concern, and the pupilphase modulator 600 may be implemented using the embodiment shown inFIGS. 29A-29B (without the connection rods 670 but with a siliconmembrane 700 extending throughout the pupil phase modulator 600) ifimage quality is a greater concern.

It is understood that the layers of the annular pattern 636 itself inthe embodiment shown in FIGS. 29A-29B may also be configured differentlythan the corresponding layers of the embodiment shown in FIGS. 26A-26B.In the embodiment shown in FIGS. 29A-29B, a 180 degree phase shift isdesired between the annular pattern 636 and the regions 660. As such,the thicknesses of the layers of the annular pattern 636 may need to bechanged slightly, to account for the additional phase shift (howeverslight it may be) caused by the silicon membrane 700. For example, thethickness of the phase-shifting layer 710 may be increased compared tothe embodiment of FIGS. 26A-26B, in order to compensate for the extraphase shift caused by the silicon membrane 700.

It is understood that the rod-free pupil phase modulator 600 (having thesilicon membrane 700 structure shown in FIGS. 29A-29B) scheme may alsoapply to other embodiments where multiple phase-shifting layers areimplemented, such as the embodiments discussed above with reference toFIGS. 28A-28C. Regardless of the specific embodiment, it is understoodthat additional reconfiguring of the thicknesses (and/or the materialcompositions) of the phase-shifting layers may be needed to compensatefor the additional phase shift introduced by the presence of the siliconmembrane in the regions 660.

Yet another alternative embodiment of the pupil phase modulator 600 isshown in FIGS. 30A-30B, where FIG. 30A illustrates a simplifieddiagrammatic planar view of the pupil phase modulator 600, and FIG. 30Billustrates a simplified diagrammatic cross-sectional side of the pupilphase modulator 600 of FIG. 30A, where the cross-section is taken frompoint A to point A′. The embodiment of the pupil phase modulator 600 inFIGS. 30A-30B is substantially similar to the embodiment shown in FIGS.29A-29B, except that the mechanical support layer 700 (i.e., the siliconmembrane 700) is also filling the gaps between the annular patterns 636.Again, the phase-shifting layer(s) may need to be reconfigured slightly(e.g., by adjusting its thickness) to compensate for the additionalphase shift attributed to the extra silicon membrane 700.

It is understood that the embodiments discussed above may offer optimalthroughput improvement for IC patterns with a low pattern density, suchas the IC pattern 160 discussed above with reference to FIG. 14. Forexample, the IC pattern 160 may include low pattern density features 162that are vias. Other examples of low pattern density features mayinclude cuts patterns with a plurality of cut features designed to forma circuit pattern (such as gates or metal lines) with one or more mainpatterns defined on corresponding mask by two or more exposures.

On reason for the improvement with respect to the low pattern density isthat the 0-th order diffraction light corresponding low pattern densityfeatures has a greater negative amplitude (i.e., more negative) than forthe high pattern density features. The greater negative amplitudeenables the 0-th order diffraction ray to be inverted and thenattenuated accordingly to cancel out the negative floor amplitude forthe higher order diffraction light. In the case of high pattern density,the 0-th order diffraction light is less negative (e.g., closer to zeroeven though it is negative), and thus even if it is phase inverted, itmay not be sufficient to cancel out the negative floor amplitude of thehigher order diffraction light rays.

Nevertheless, it is understood that the present disclosure may still beincorporated for high pattern density features to improve throughput andimage quality, though these improvements may be more significant for lowpattern density features (e.g., having a pattern density less than 20%or 10%). In addition to throughput improvements, the pupil phasemodulator 600 of the present disclosure may also offer MEEF enhancementsand reduced printability of particles on the mask for reasons similar tothose discussed above with reference to FIGS. 14-18.

FIG. 31 is a flowchart illustrating a method 800 of performing anextreme ultraviolet lithography (EUVL) process to a wafer.

The method 800 includes a step 810 of loading an EUV mask to alithography system.

The method 800 includes a step 820 of directing, using an illuminator,an EUV light to the EUV mask. The EUV mask diffracts the EUV light intoa 0-th order ray and a plurality of higher order rays.

The method 800 includes a step 830 of inverting a phase of the 0-thorder ray using a pupil phase modulator. The phase modulator includesone or more phase-shifting layers having different material compositionsfrom one another and one or more mechanical support layers configured toprovide mechanical support to the one or more first phase-shiftinglayers. In some embodiments, the one or more phase-shifting layerscontain molybdenum, zirconium, ruthenium, or nickel, and the one or moremechanical support layers each contain silicon. The pupil phasemodulator is positioned in a pupil plane that is located between the EUVmask and the wafer. The pupil phase modulator has a planar view thatsubstantially corresponds with a planar view of the illuminator.

The method 800 includes a step 840 of performing a lithography exposureprocess to the wafer at least in part using a phase-inverted 0-th orderray.

In some embodiments, the higher order rays have a negative flooramplitude, and a phase-inverted 0-th order ray has a positive amplitude.The method 800 may further include a step of attenuating, using thepupil phase modulator, an amplitude of the 0-th order ray such that thepositive amplitude of the phase-inverted 0-th order ray substantiallycancels out the negative floor amplitude of the higher order rays.

One embodiment of the present disclosure includes a lithography system.The lithography system includes a radiation source configured togenerate an extreme ultraviolet (EUV) light. The lithography systemincludes a mask that defines one or more features of an integratedcircuit (IC). The lithography system includes an illuminator configuredto direct the EUV light onto the mask. The mask diffracts the EUV lightinto a 0-th order ray and a plurality of higher order rays. Thelithography system includes a wafer stage configured to secure a waferthat is to be patterned according to the one or more features defined bythe mask. The lithography system includes a pupil phase modulatorpositioned in a pupil plane that is located between the mask and thewafer stage. The pupil phase modulator is configured to change a phaseof the 0-th order ray.

Another embodiment of the present disclosure includes a pupil phasemodulator. The pupil phase modulator includes a phase-shifting layerconfigured to shift a phase of a 0-th order ray. The 0-th order ray isone of a plurality of rays that are diffracted by an extreme ultraviolet(EUV) mask in response to an incident EUV light directed onto the EUVmask. The pupil phase modulator includes one or more mechanical supportlayers configured to provide mechanical support for the phase-shiftinglayer.

Yet another embodiment of the present disclosure includes a method ofperforming an extreme ultraviolet lithography (EUVL) process to a wafer.An EUV mask is loaded to a lithography system. Using an illuminator, anEUV light is directed to the EUV mask. The EUV mask diffracts the EUVlight into a 0-th order ray and a plurality of higher order rays. Aphase of the 0-th order ray is inverted using a pupil phase modulator.The pupil phase modulator being positioned in a pupil plane that islocated between the EUV mask and the wafer. The pupil phase modulatorhas a planar view that substantially corresponds with a planar view ofthe illuminator. A lithography exposure process is performed to thewafer at least in part using a phase-inverted 0-th order ray.

The foregoing outlines features of several embodiments so that thoseskilled in the art may better understand the aspects of the presentdisclosure. Those skilled in the art should appreciate that they mayreadily use the present disclosure as a basis for designing or modifyingother processes and structures for carrying out the same purposes and/orachieving the same advantages of the embodiments introduced herein.Those skilled in the art should also realize that such equivalentconstructions do not depart from the spirit and scope of the presentdisclosure, and that they may make various changes, substitutions, andalterations herein without departing from the spirit and scope of thepresent disclosure.

What is claimed is:
 1. A lithography system, comprising: a radiationsource configured to generate radiation; a mask that defines one or morefeatures of an integrated circuit (IC); an illuminator configured todirect the radiation onto the mask, wherein the illuminator includes afirst pattern; a wafer stage configured to hold a wafer; and a pupilphase modulator located between the mask and the wafer stage, whereinthe pupil phase modulator includes a second pattern, wherein the secondpattern is defined by a mechanical support layer and a plurality ofphase-shifting layers disposed over the mechanical support layer.
 2. Thelithography system of claim 1, wherein: the plurality of phase-shiftinglayers includes a first phase-shifting layer having a first materialcomposition and a second phase-shifting layer having a second materialcomposition different from the first material composition.
 3. Thelithography system of claim 2, wherein: the first material compositionincludes molybdenum; and the second material composition includeszirconium or nickel.
 4. The lithography system of claim 2, wherein: themechanical support layer is a first mechanical support layer; the pupilphase modulator further includes a second mechanical support layer; thefirst phase-shifting layer is disposed between the first mechanicalsupport layer and the second phase-shifting layer; and the secondphase-shifting layer is disposed between the first phase-shifting layerand the second mechanical support layer.
 5. The lithography system ofclaim 2, wherein the plurality of phase-shifting layers further includesa third phase-shifting layer having a third material composition that isdifferent from the first material composition and from the secondmaterial composition.
 6. The lithography system of claim 1, wherein: thepupil phase modulator includes a first region and a second region; thephase-shifting layers are located in the first region but not the secondregion; and the mechanical support layer is located in both the firstregion and the second region.
 7. The lithography system of claim 1,wherein: the phase-shifting layers are opaque with respect to theradiation generated by the radiation source; and the mechanical supportlayer is transparent with respect to the radiation generated by theradiation source.
 8. The lithography system of claim 1, wherein theradiation source is configured to generate Extreme Ultraviolet (EUV)light as the radiation.
 9. A pupil phase modulator, comprising: amechanical support layer; a first phase-shifting layer disposed over themechanical support layer, wherein the first phase-shifting layer isconfigured to shift a phase of a light that is diffracted by alithography mask; and a second phase-shifting layer disposed over thefirst phase-shifting layer, wherein the second phase-shifting layer isconfigured to further shift the phase of the light, and wherein thefirst phase-shifting layer and the second phase-shifting layer havedifferent material compositions.
 10. The pupil phase modulator of claim9, wherein: the first phase-shifting layer contains molybdenum; and thesecond phase-shifting layer contains zirconium or nickel.
 11. The pupilphase modulator of claim 9, wherein the mechanical support layer istransparent.
 12. The pupil phase modulator of claim 9, wherein themechanical support layer includes a silicon membrane.
 13. The pupilphase modulator of claim 9, wherein the first phase-shifting layer andthe second phase-shifting layer collectively define an opaque pattern ina top view.
 14. The pupil phase modulator of claim 13, wherein theopaque pattern has an annular shape or a quasar shape in the top view.15. The pupil phase modulator of claim 9, wherein: the mechanicalsupport layer is a first mechanical support layer; the pupil phasemodulator further comprises a second mechanical support layer; and thefirst phase-shifting layer and the second phase-shifting layer aredisposed between the first mechanical support layer and the secondmechanical support layer.
 16. The pupil phase modulator of claim 9,further comprising a third phase-shifting layer disposed over the secondphase-shifting layer, wherein the third phase-shifting layer isconfigured to further shift the phase of the light, and wherein thethird phase-shifting layer has a different material composition than thefirst phase-shifting layer and the second phase-shifting layer.
 17. Thepupil phase modulator of claim 9, wherein the first phase-shifting layerand the second phase-shifting layer is laterally surrounded by vacuum.18. A method of performing a lithography process to a wafer, comprising:loading a lithography mask to a lithography system; directing, using anilluminator, a light in an extreme ultraviolet (EUV) range to thelithography mask, wherein the illuminator includes a first pattern; andshifting at least a phase of at least a portion of the light using apupil phase modulator, the pupil phase modulator including a secondpattern that corresponds with the first pattern in a top view, whereinthe second pattern is defined by a first phase-shifting layer located ona mechanical support layer and a second phase-shifting layer locatedover the first phase-shifting layer, wherein the first phase-shiftinglayer and the second phase-shifting layer have different materialcompositions.
 19. The method of claim 18, wherein: the first patterntransmits the light; and the second pattern blocks the light.
 20. Themethod of claim 18, wherein the portion of the light is a 0-th order raythat has been diffracted by the lithography mask, and wherein theshifting comprises shifting the phase of the 0-th order ray.